1. Field of the Invention
The invention relates to a method of synthesizing single diamond crystals of high thermal conductivity and having at least 99.9 atomic % carbon-12. The method involves graphitizing amorphous carbon of at least 99.9 atomic % carbon-12 to form a highly crystalline carbon which can then be submitted to high temperature and pressure in a temperature difference process to form synthetic single diamond crystals.
2. Description of Related Art
Diamond has the highest thermal conductivity of all known materials and because of this property, diamond is commonly used as a heat sink in high power lasers or as a window material in CO.sub.2 lasers, both of which require good heat conduction.
However, carbon which is the constituent element of diamond inherently contains approximately 1.1 atomic % of an isotope with mass number 13, so the thermal conductivity of naturally occurring diamond is lower than that of the ideal one which is solely composed of carbon atoms having mass number 12 exhibiting a value of only approximately 20 W/cm.multidot.K at room temperature. If a perfect diamond composed solely of carbon atoms having mass number 12 were produced, it is supposed that it would exhibit a thermal conductivity of at least 50 W/cm.multidot.K. (See Diamond Research pages 7-13 (1976)).
It is known that diamond can be synthesized by subjecting a carbon source and a metal solvent to ultra high pressure and high temperature conditions wherein diamond is thermodynamically stable. A temperature difference process is a synthesis method of choice that can produce large and excellent diamond crystals.
The general layout of a sample cell used in the practice of this method is shown in FIG. 4. The sample cell in an ultra high pressure apparatus contains a carbon source 1 and a diamond seed crystal 2 between which a metal solvent 3 such as Fe, Ni or Co is held. While an ultra high pressure is exerted via a pressure medium 5, a heater 4 is activated to create a temperature difference of several tens of degrees in Celsius between a hotter portion of the carbon source 1 and a cooler portion of the seed crystal 2. The carbon dissolved in the metal solvent 3 is transported to the cooler portion and subjected to pressure and temperature conditions where diamond is thermodynamically stable. By this means, single diamond crystals are grown on the seed crystal 2 supported on a seed bed 6.
In order to insure that single diamond crystals of high thermal conductivity that are solely composed of carbon atoms with mass number 12 can be synthesized by the temperature difference process and other methods of diamond synthesis, carbon with mass number 12 must be used as a carbon source.
Carbon with mass number 12 is conventionally produced by a process that comprises separating the desired carbon from carbon monoxide or methane gas by mass separation and then carbonizing the separated carbon. The purity of the product is approximately 99.9-99.99 atomic %. However, the carbon with mass number 12 that is obtained by this process is amorphous with low crystallinity and consists of fine grains.
Attempts have already been made to synthesize a diamond of high thermal conductivity using as a carbon source said amorphous carbon containing at least 99.9 atomic % of carbon having a mass number 12. However, the crystals grown by using amorphous carbon contain more metal inclusions and irregular shape crystals than crystals grown from natural and artificial graphites that have higher degrees of crystallinity at the same grain size. Thus, it is difficult to obtain satisfactory crystals with the amorphous carbon. A probable explanation for this phenomenon would be that the low crystallinity of the carbon source affects the rate of its dissolution into the metal solvent and makes it impossible for the carbon to be transported at an appropriate speed. Another problem with the amorphous carbon is that it easily adsorbs various impurity gases that will contaminate the atmosphere of diamond synthesis.
A compacted carbon disk or compact is conventionally used as the carbon source but the particles of the amorphous carbon containing at least 99.9 atomic % of carbon with mass number 12 are so small that the density of the compact can only be increased to approximately 1.1 g/cm.sup.3 which is about one half the density of the compact of a natural graphite powder (2.0 g/cm.sup.3). As a consequence, the compact placed under ultra high pressure diamond synthesis conditions has a smaller thickness than that of a compact made from ordinary graphite powder. This causes smaller thickness variations in the temperature profile not only in the direction in which carbon is dissolved but also in its transport direction. These variations in the temperature profile make it impossible to achieve an appropriate speed of carbon transport and, hence, satisfactory diamond crystals (as were achieved in the case described above) cannot be obtained. Further, this low density compact requires an excessively large applied load in order to generate pressure of at least 55 Kb necessary to produce a stable diamond. This shortens the life of synthesis equipment and increases the cost of diamond production.